Your MPSC/SPMC/MPMC queue is not a queue

Lockless queues let multiple cores communicate with each other without mutexes, typically to move work around for parallel processing. They come in four variants: {single,multi}-producer {single,multi}-consumer. A producer gives data to a consumer, each of which can be limited to a single thread (i.e. a single-{producer,consumer}) or shared across multiple threads. But only the single-producer single-consumer (SPSC) queue is actually a queue!